Molecular cloning and heterologous expression of human neurotransmitter receptors provides a means to study these important targets of drug action in isolation. Transfected cell lines which express single human receptor subtypes provide excellent experimental model systems, free of the many closely related receptor subtypes which confuse the analysis of data in complex neural tissues. Heterologous expression systems can be used as living cell assay systems for the determination of receptor activation of inhibition. Since both drug efficacy and drug affinity play central roles in the actions of therapeutic agents, high-throughput functional screening assays for single human receptor subtypes will provide important new tools for drug development. The overall goal of this project is to develop a set of sensitive, high-throughput assay systems which measure the functional activation of cloned human G protein-coupled receptors in cultured mammalian cells. In Phase I of this project, the applicants have successfully developed a fluorescence assay for receptor-mediated adenylate cyclase inhibition in mammalian cells transfected with the human serotonin 5-HT (1A) receptor gene. In Phase II, they intend to complete the characterization of this system, to optimize the assay methods for use in high-throughput functional drug screening, and to diversify our program by transfecting into our cell lines other cloned human receptors which stimulate or inhibit adenylate cyclase activity.